J* is a dynamic embeddable scripting language designed to be as easy as possible to embed into
another program. It arises from the need of having a modern scripting language with built-in
support for OOP whilst mantaning simplicity of use and a low memory footprint. It can be viewed as
a middle ground between Python, a more complete scripting language with lots of features and
libraries, and Lua, a small and compact language that is simple to embed but doesn't provide OOP
functionalities out of the box.
J* tries to take the best of both worlds, implementing a fully featured class system while
maintaining a small footprint and employing the use of a stack based API for communication
among the language and host program, rendering embedding simple.
J* is:
- Small. The implementation spans only a handful of files and the memory footprint is low thanks to a minimal standard library that provides only essential functionalities
- Easy to use. The API is contained in a single header file and employs a stack based approach similar to the one of Lua, freeing the user from the burden of keeping track of memory owned by the language
- Fully object oriented. Every entity, from numbers to class instances, is an object in J*
- Modular. A fully fledged module system makes it easy to split your code across multiple files
- Easily extensible. The language can be easily extended by creating C functions callable from J* using the API, or by importing C extensions provided as dynamic libraries.
To get a feel of the language, try it in your browser!
Besides the language implementation, a simple command line interface called jstar
is provided to
start using the language without embedding it into another program.
If the jstar
binary is executed without
arguments it behaves like your usual read-eval-print loop, accepting a line at a time and executing
it:
J*>> var helloWorld = 'Hello, World!'
J*>> print(helloWorld)
Hello, World!
J*>> _
You can even write multiline code, it will look like this:
J*>> for var i = 0; i < 3; i = 1
.... print('Hello, World!')
.... end
Hello, World!
Hello, World!
Hello, World!
J*>> _
When you eventually get bored, simply press Ctrl d or Ctrl c to exit the interpreter.
If you instead want to execute code written in some file, you can pass it as an argument to jstar
.
All arguments after the first will be passed to the language as script arguments, you can then read
them from the script this way:
if #argv > 0
print("First argument: ", argv[0])
else
raise Exception("No args provided")
end
The jstar
executable can also accept various options that modify the behaviour of the command line
app. To see all of them alongside a description, simply pass the -h
option to the executable.
In addition to being a useful tool to directly use the programming language, the command line interface is also a good starting point to learn how J* can be embedded in a program, as it uses the API to implement all of its functionalities. You can find the code in apps/jstar/.
Another application, called jstarc
, is provided alongside the cli and the language runtime. As the
name implies, this is a compiler that takes in J* source files, compiles them to bytecode
and stores them on file.
Below is a typical usage of jstarc
:
jstarc src/file.jsr -o file.jsc
You can even pass in a directory if you want to compile all jsr
files contained in it:
# This compiles all *.jsr files in `dir` and stores them in a directory `out`
# Both directories have to exist
jstarc dir/ -o out/
The output .jsc
files behave in the same way as normal .jsr
source files. You can pass them
to the jstar
cli app to execute them and can be even imported by other J* files.
Compiled files are not faster to execute than normal source files, as the J* vm will always compile source to bytecote before execution, but have nonetheless some nice advantages:
- Compactness. compiled files are more compact than source files and generally take up less space
- Faster startup. Reading a compiled file is orders of magnitude faster than parsing and compiling source code, so there's almost no delay between importing and actual execution
- Obfuscation. If you don't want your source to be viewed, compiled files are a nice option since all the source and almost all debug information are stripped
- Platform indipendence. Compiled files are cross-platform, just like normal source files. This means that they can be compiled once and shared across all systems that have a J* interpreter.
Check out the Pulsar static analyzer for code linting and
static analysis from the command line.
Check the VSCode J* extension
for linting and syntax highlighting support in VSCode.
Special thanks to Bob Nystrom and the invaluable crafting interpreters book, on which the VM is based.
My gratitude goes to the Lua project as well, for inspiring the stack-based
C API and its amazing pattern matching library, on which the
re
module is based on.
Also, the closures in Lua
and implementation of Lua 5 articles were crucial for some
parts of the implementation.
The J* library requires a C99 compiler, CMake (>= 3.9) and Python (>= 2.7) to be built, and is known to compile on OSX (Apple clang), Windows (both MSVC and MinGW-w64) and Linux (GCC, clang).
To build the provided command line interface jstar
, a C 11 compiler is required as one of its
dependencies, is written in C .
You can clone the latest J* sources using git:
git clone --recurse-submodules https://github.com/bamless/jstar.git
After cloning, use CMake to generate build files for your build system of choice and build the all
target to generate the language dynamic/static libraries and the command line interface. On
UNIX-like systems this can be simply achieved by issuing this in the command line:
cd jstar; mkdir build; cd build; cmake ..; make -j
Once the build process is complete, you can install J* by typing:
sudo make install
Various CMake options are available to switch on or off certain functionalities of the interpreter:
Option name | Default | Description |
---|---|---|
JSTAR_NAN_TAGGING | ON | Use the NaN tagging technique for storing the VM internal type. Decrases the memory footprint of the interpreter and increases speed |
JSTAR_COMPUTED_GOTOS | ON | Use computed gotos to implement the VM eval loop. Branch predictor friendly, increases performance. Not all compilers support computed gotos (MSVC for example), so if you're using one of them disable this option |
JSTAR_INSTALL | ON | Generate install targets for the chosen build system. Turn this off if including J* from another CMake project |
JSTAR_SYS | ON | Include the 'sys' module in the language |
JSTAR_IO | ON | Include the 'io' module in the language |
JSTAR_MATH | ON | Include the 'math' module in the language |
JSTAR_DEBUG | ON | Include the 'debug' module in the language |
JSTAR_RE | ON | Include the 're' module in the language |
JSTAR_DBG_PRINT_EXEC | OFF | Trace the execution of instructions of the virtual machine |
JSTAR_DBG_STRESS_GC | OFF | Stress the garbage collector by calling it on every allocation |
JSTAR_DBG_PRINT_GC | OFF | Trace the execution of the garbage collector |
JSTAR_INSTRUMENT | OFF | Enable instrumentation timers scattered throughout the code. Running J* will then produce 3 json files importable from chrome://tracing to view a timeline of executed functions. Supported only when using the GCC compiler on POSIX systems |
Precompiled binaries are provided for Windows and Linux for every major release. You can find them here.